Polyurethane Elastomers with Monodisperse Segments and their Model Precursors: Synthesis and Properties

Abstract

Uniform model compounds for the soft and hard segment of polyurethane (PU) elastomers and the corresponding tailormade elastomers with monodisperse segment length distribution were synthesized and characterized with the objective of getting a better understanding of the structure and morphology as well as the structure-property relationships of multiphase segmented PU elastomers from the study of such clearly defined model systems.

Monodisperse α-hydro-ω-hydroxypoly(oxytetramethylene) and hydroxy-terminated oligomers of 1,4-butanediol and 4,4’-diphenylmethanediisocyanate (soft and hard segment precursors) as well as their bis-(diphenylmethaneurethanes) (soft and hard segment model compounds) were obtained in a stepwise synthesis and in combination with chromatographic fractionation techniques, and in some cases (oligourethanes) by employing the tetrahydropyranyl protecting group. The hydroxy-terminated segment precursors were reacted to the PU elastomers with strictly mono-disperse hard segments and narrow soft segment length distribution by a modified prepolymer process which largely suppresses the pre-extension reaction; it was established that the pre-polymer formation in the melt doesn’t obey Flory statistics and that the deviations also vary with the molecular weight of the starting polyether.

The soft segment model compounds with terminal urethane groups exist in two crystalline modifications with different hydrogen bond strength between neighbouring urethane groups. Polymorphism is also observed in the case of the hydroxy-terminated hard segment precursors and is connected with deviations from the planar zig-zag conformation of the butanediol urethane part. There is no evidence for chain folding, and cocrystallization of oligourethanes of varying length only occurs between oligomers of successive length and not until a critical chain length of three repeat units is exceeded. Furthermore, the urethane group has been found to undergo a rapid transurethanization (urethane interchange) reaction already in the oligourethane melting temperature range and even without any catalyst, rendering monodisperse into polydisperse systems. Consequently, the hard segment length distribution and thus the hard domain morphology of segmented PU elastomers are affected by the thermal history in that the usual processing temperatures result in an equilibration of the segment length distribution and thus a reorganization of the hard segments.

PU elastomers with a narrow (monodisperse) hard segment length distribution show a better phase separation and higher degree of crystallinity in the hard domains as compared to elastomers with broad segment length distribution, even though the structure and morphology of the semicrystalline hard domaines is still complex and not yet fully understood. Concerning the differences in the mechanical properties between elastomers with narrow and broad segment length distributions it can be stated that monodisperse hard segments generally result in improved properties, e.g., a higher and flatter plateau modulus over a wider temperature range, and a higher softening temperature as compared to elastomers with polydisperse hard segment length distribution are characteristic for the dynamic mechanical behaviour; the soft segment length distribution has a comparatively minor effect and only the average soft segment length is of some importance in connection with the possibility of soft segment crystallization.